There is a Jali in the CAT , if that is Removed or Broken then you need to replace the Cat. My Steermatic Cat Jali was broken and Avg was Dropped, i had to Replace it and now i am getting again 10 to 12 in City with AC, 1.5 CC.
CAT Is very important Element of car, it helps to normalize the GASSES which is suitable for Enviorment and it helps to gain the Compression of Engine too,
if you pass Gas emission by Silencer without CAT, your Damanging the Enviorment and Hurting your engine Compression too. it will effect your car Pefofrmance and lower your Engine life.
I replaced the Cat for City 2007 Steermatic cost me 9 to 10 k in karachi. Abul hassan Ispahni Road. CAT Is very important.